PETROLOGY AND GEOCHEMISTRY OF THE LATE PALEOZOIC VOLCANIC ROCKS IN THE WEST TIANSHAN, NW CHINA

The Yili terrane is sandwiched within the north Tianshan and southwestern Tianshan orogenic belts. Devonian – Carboniferous mafic - intermediate volcanites, mainly consisting of basalt, trachyte, trachy-andesite, andesite, and rhyolite, crop out along the north edge of the Yili terrane (NEY) and the south edge of the Yili terrane (SEY). This work studies these Late Paleozoic volcanic rocks and discusses their geological significance.

The time span of the studied volcanic rocks is quite large (>380Ma to 310Ma) based on zircon U-Pb data. The magmatic activity in the western part of the west Tianshan was earlier (Devonian) than that in the eastern part (Carboniferous). The variation in time and space of the volcanic rocks in the west Tianshan indicates that the magma eruptions started at the western part during the Devonian. Volcanic activity in the middle part occurred at Early Carboniferous. On the eastern part, the magmatic eruptions did not occur until the Late Carboniferous, which mostly formed trachy-andesite, andesite, trachyte, and rhyolite. This time-space trend suggests a volcanic chain, progressing from the western to the eastern part of the west Tianshan.

Geochemistry demonstrates that the studied volcanic rocks are typical of continental arc magmatism. The differences between the Sr-Nd-Hf isotope compositions of the basaltic and andesitic rocks rule out one origin of each whole assemblage by simple fractional crystallization and strongly support the occurrence of open-system processes for the Devonian volcanic rocks. The rocks with low e_Nd and high initial ⁸⁷Sr/⁸⁶Sr values should be produced by a combination of continental crust and mantle. The Devonian - Carboniferous volcanic rocks distributing along the SEY represent the continental arc formed during the subduction of the Paleo- South Tianshan Ocean (PSTO). The PSTO continued its subduction northwards during Devonian to Late Carboniferous. In the same way, volcanic rocks on the NEY represent the continental margin of the Yili terrane formed during the subduction of the Junggar Ocean southwards.